1. Field of the Invention
The present invention relates to a reception apparatus for a radio communication system which utilizes a minimum preamble signal to assure a high efficiency in high speed transmission in a multi-path environment, and more particularly to a technique for high speed equalization processing and a technique for reduction of the power consumption.
2. Description of the Related Art
In data transmission of a high speed radio ATM (Asynchronous Transfer Mode) system which is one of multimedia mobile communication systems of 20 to 30 Mbps (megabits/second: unit of the transmission rate) and uses the 5.2 GHz band, in order to prevent quality deterioration of data by multi-path fading, an equalization function is used, and in order to allow high speed processing of an equalizer, a minimum preamble signal is used.
As an apparatus of the type described, for example, a radio data communication terminal of the narrow-band modulation type is disclosed in Japanese Patent Laid-Open No. 308158/1999 which can use a minimum preamble to determine a frequency offset value for operating a phase rotating element and then set a tap coefficient to be used by an equalizer. More particularly, the radio data communication terminal determines a frequency offset value for operating an automatic frequency control circuit within a preamble period of one frame period in accordance with a narrow-band modulation system wherein NRZ (Non-Return to Zero) codes of the opposite polarities of the GMSK (Gaussian filtered MSK (Minimum Shift Keying)) are passed through a low-pass filter of the Gaussian type and then inputted to a phase-continuous FSK (Frequency Shift Keying) modulator of a modulation index of 0.5 to modulate the codes, estimates the transmission line characteristic, determines a tap coefficient necessary for an equalizer, sets the tap coefficient to the equalizer and then performs equalization of a reception signal by means of the equalizer.
FIG. 4 shows an example of a configuration of an equation function of a conventional radio reception apparatus which uses a minimum preamble to determine a frequency offset value for operating a phase rotating element and then sets a tap coefficient to be used by an equalizer.
Referring to FIG. 4, the radio reception apparatus shown includes two first and second antennae 8a and 8b, a radio frequency (RF) section 9, a carrier detection section 10, an equalization processing section 11. The equalization processing section 11 includes a memory section 12, a phase rotating section 13, a phase difference detection section 14, an average value detection section 15, an integration circuit 16, a vector conversion circuit 17, a transmission line characteristic estimation section 18, a tap coefficient setting section 19, and an equalizer 20.
Transmission data from a base station are received by the two antennae 8a and 8b. The RF section 9 receives the reception data from the antennae 8a and 8b, performs a frequency conversion process and outputs the reception data of the converted frequency (quadrature demodulated I and Q signals) to the equalization processing section 11. The RF section 9 outputs a received signal strength indicator (RSSI) signal Q to the carrier detection section 10.
The carrier detection section 10 discriminates presence/absence of a carrier based on the RSSI signal Q from the RF section 9, and outputs, to the equalization processing section 11, a carrier sense signal S which exhibits an active state when a start of receive data is detected. Further, the carrier detection section 10 receives a demodulation data end signal R of a one-pulse signal representative of an end of demodulation data supplied thereto and outputs, to the equalization processing section 11, the carrier sense signal S serving as a control signal for stopping the outputting of demodulation data from the equalization processing section 11.
The memory section 12 fetches a reception data signal P (quadrature demodulation signal after conversion into a digital signal by an A/D conversion section not shown) for an arbitrary period of time and controls the outputting.
The phase rotating section 13 rotates the phase of the output signal of the memory section 12 by a necessary amount. The phase difference detection section 14 determines an angle at present and another angle after one period of a PN (Pseudo Noise) code string and determines a difference between the angles.
The average value detection section 15 integrates the value of the angle difference determined by the phase difference detection section 14 for a predetermined number of times and then divides the integrated value by the number of times to determine an average value (frequency offset value) of an average phase difference per one symbol. The integration circuit 16 integrates the frequency offset value determined by the average value detection section 15 in a unit of a symbol. The vector conversion circuit 17 converts a signal outputted from the integration circuit 16 into a real part amplitude value and an imaginary part amplitude value and outputs the real part amplitude value and the imaginary part amplitude value to the phase rotating section 13. The transmission line characteristic estimation section 18 uses the signal after the phase rotation by the phase rotating section 13 to determine a transmission line characteristic for one period of the PN code string within the preamble period. The tap coefficient setting section 19 determines a tap coefficient necessary for the equalizer 20 from the transmission line characteristic determined by the transmission line characteristic estimation section 18 and sets the tap coefficient to the equalizer 20. The equalizer 20 equalizes the output of the phase rotating section 13 by means of a filter having the tap coefficient set by the tap coefficient setting section 19 and outputs a demodulation data signal T to effect a reception process.
FIG. 5 illustrates an example of operation timings in an equalization function process of the radio reception apparatus shown in FIG. 4. Referring to FIG. 5, within an antenna changeover selection period δ of a preamble signal period γ positioned preceding to an information data period, the integration is performed on the antenna 8a side for a certain fixed period for each one frame, and then the antenna to be used is changed over to the antenna 8b. After the changeover, the integration is performed on the antenna 8b side for another certain fixed period. The integration output values integrated for the first antenna 8a side and the second antenna 8b side are compared with each other to select the antenna which exhibits a higher reception level. Then, the antenna to be used is fixed to the selected antenna, and burst reception (reception of information data) is performed using the selected antenna.
Within the preamble signal period γ, the carrier detection section 10 discriminates presence/absence of a carrier to detect a start of reception data, and then automatic gain control (AGC) and automatic frequency control (AFC) by an automatic frequency control circuit (not shown) for dealing with amplitude and phase variations in demodulation processing are performed. Further, the equalization processing section 11 performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient.
FIG. 6 illustrates an example of reception timings of the conventional radio reception apparatus shown in FIG. 4 when an idle time is comparatively long. The reception data signal P successively received from the RF section 9 is composed of a preamble signal used for various kinds of training and information data. Within a preamble period placed before an information data period within one frame period, the same PN string is transmitted repetitively. The carrier detection section 10 discriminates presence/absence of a carrier based on the RSSI signal Q from the RF section 9 to detect a start of reception data, and after a start of reception data is detected, that is, after the carrier sense signal S outputted from the carrier detection section 10 changes into an active state, the equalization processing section 11 performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient.
In the initialization of the equalizer 20, a preamble signal which includes repetitions of a PN code is stored into the memory section 12 and processed for a certain fixed time, and therefore, a delay appears as much. The demodulation data signal T is outputted after the initialization of the equalizer 20. As seen from FIG. 6, a delay corresponding to the initialization period of the equalizer 20 occurs at the equalization processing section 11, and the transmission efficiency is deteriorated because the idle period is long.
FIG. 7 illustrates an example of reception timings of the conventional radio reception apparatus shown in FIG. 4 when the idle period is short. The reception data signal P successively received from the RF section 9 is composed of a preamble signal used for various kinds of training and information data. After presence/absence of a carrier is discriminated based on the RSSI signal Q from the RF section 9 to detect a start of reception data, the equalization processing section 11 performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient. In the initialization of the equalizer 20, a preamble signal which exhibits repetitions of a PN code is stored into the memory section 12 and processed for a certain fixed period of time. Therefore, a delay for approximately 200 symbols in the maximum occurs. The demodulation data signal T is outputted after the initialization of the equalizer 20 is performed.
If the idle period is shorter than the delay and a next frame is received within a carrier sense period ε, then the carrier sense signal ζ at a rising edge cannot be detected due to a collision of the frames. Reception data for one frame within which the carrier sense signal ζ is not successfully detected at a rising edge cannot be received normally, and a miss of one frame occurs with the demodulation data signal.
The conventional reception apparatus shown in FIG. 4 cannot perform high speed reception since the preamble signal is longer by the antenna changeover selection period δ of the preamble signal period γ. On the other hand, where the idle period from the end of information data to the start of the preamble period of the next frame is short, reception data cannot be received normally.
In the conventional radio reception apparatus described with reference to FIGS. 4 to 6, integration output values integrated for the antenna 8a side and the antenna 8b side are compared with each other to select that one of the antennae which exhibit a higher reception level (reception sensitivity) within the antenna changeover selection period δ within the preamble signal period γ within which various kinds of training are performed, and then AGC and AFC as well as initialization necessary for the equalizer are performed. Therefore, the preamble signal becomes longer by the antenna changeover selection period δ, and this deteriorates the transmission efficiency. Then, where the idle period is comparatively long, the transmission efficiency is deteriorated. On the other hand, where the idle period is comparatively short, if the idle period is shorter than a delay time and a next frame is received within a processing period of demodulation data, then a carrier sense signal cannot be detected. Consequently, such a problem occurs that reception data are received but abnormally for every other frame.